Paper and tissue are typically manufactured in a continuous sheet on a papermaking machine. One of the most common papermaking machines is the Fourdrinier machine. Fourdrinier machines generally include at least three sections: a wet-end section, a press section, and a dryer section. The wet-end section, which can be 40 to 100 feet in length, is also referred to as the forming section or the Fourdrinier table. In the wet-end section, stock flow is transferred from a headbox onto a moving, endless belt of wire-mesh screen, referred to as the Fourdrinier wire, or simply as the “wire.” Stock flow is normally a combination of wood fibers, fines and fillers, chemical additives such as bonding agents, and water. Wood fibers typically range in length from 400 to 7,000 microns and in width from 20 to 100 microns, depending on the species of the wood. Stock flow typically has a liquid consistency of 99 percent and a fiber consistency of approximately 0.2 to 1 percent (although other fiber consistencies are possible), depending on the grade and weight of the paper or tissue being manufactured.
The function of the headbox is to distribute stock flow with a uniform fiber distribution to the wire in order to produce a sheet of paper having uniform properties across the width of the wire (cross-machine direction), along the length of the wire (machine direction), and through the cross-section of the sheet of paper (Z direction). The headbox distributes stock flow to the wire at an angle other than absolute tangent, referred to as the angle of impingement. If the angle of impingement is steep, i.e., close to absolute tangent, the arrangement of the headbox is referred to as pressure forming. If the angle of impingement is shallow, i.e., not close to absolute tangent, the arrangement of the headbox is referred to as velocity forming.
The wire runs over a breast roll, which is usually located under the headbox. The wire is typically not a permanent part of the papermaking machine and requires periodic replacement. One condition leading to premature failure of the wire is the plugging of the openings in the porous wire by the fibers, fines, and fillers of the web being transported by the wire. Normally, the wire is a delicate, finely woven metal or synthetic fiber cloth that allows for drainage of the water, but retains most of the fibers. The strands of the wire are commonly made of finely drawn and woven, annealed bronze or brass.
After the stock flow is delivered from the headbox to the wire in the wet-end section of a Fourdrinier machine, the fibers are initially held in free suspension within the water as relatively mobile individual fibers or as part of a network, referred to as a floc. The fibers and flocs in the stock flow begin to form a wet sheet of matted pulp, referred to as an embryonic web. While not subscribing to any particular manner in which the embryonic web is formed, normally either bonding agents in the stock flow cause an electro-chemical bond or the bond is produced through physical entanglement. The embryonic web forms as the fibers and flocs in free suspension begin to settle in layers on the wire. Ideally, the fiber distribution within the web would be consistent in the cross-machine direction, the machine direction, and the Z direction. However, due to gravitational forces, the bottom-most layers of fibers that settle directly on the wire are typically more dense than the upper-most layers of fibers. The web normally has boundary layers (i.e., the two external layers of the web, such as the bottom-most layer of fibers that settles directly on the wire and the upper-most layer of fibers) and internal web fibers (fibers in the layers of the web between the two external layers of fibers). The web may consist of approximately 2 to 100 layers of fibers.
In order to assist in the formation of the embryonic web, as the wire moves away from the headbox, various suction devices can be used to drain water from the stock flow. The suction devices in the Fourdrinier machine typically include a series of stationary blades or foils. The stationary foils remove water from the stock flow by creating a vacuum on the downstream side of the blade where the wire leaves the blade surface. As the wire moves across a series of stationary foils, the downstream side of each stationary foil creates a vacuum that pulls water from the stock flow, while the upstream side of each stationary foil pulls the water off of the wire. Some of the wood fibers, fines, and fillers are pulled off of the wire along with the water being pulled off of the wire. The amount of fibers, fines, and fillers that are retained on the wire while the water is being pulled off of the wire is referred to as retention.
Once the wire passes over the stationary foils, the wire normally passes over a drive roll or couch roll, over a series of return rolls, and back to the breast roll. At the end of the wet-end section of the Fourdrinier machine, the web can have a water consistency of approximately 80 percent and a fiber consistency of approximately 20 percent. At this point, the web can normally support its own weight. Other water and fiber consistencies are also possible at this point for enabling the web to support its own weight.
Next, the web can be transferred from the wet-end section of the Fourdrinier machine to the press section at the couch roll. The wet web of paper is normally transferred from the wire of the wet-end section to a screen. The screen can be a woolen felt screen, referred to as a felt, acting as a conveyor belt to carry the web through the press section. The felt is typically porous media that provides space and channels for water removal. The felt can also act as a textured cushion or shock absorber for pressing the moist web without crushing the web. The texture and character of the felt varies according to the grade of the paper being made. The felt normally carries the web through two or more press rolls, which mechanically squeeze water from the web. A variety of suction devices, one of which is commonly referred to as a uhle box, can also be used to remove water from the felt. The press rolls often consist of a steel or cast iron core covered by a bronze or stainless steel inner shell and an outer rubber shell. At the end of the press section of the Fourdrinier machine, the web typically has a consistency of approximately 40 percent water and 60 percent fiber, although other web consistencies at this stage are possible.
After the press section, the web can be transferred to fabric dryer felts that carry the web through the dryer section. The dryer felts are most commonly constructed of a highly permeable cotton blend or open-mesh fabric. The web is normally held firmly against a number of steam-heated cylinders or drums by the dryer felts in order to evaporate the remaining water. As the web passes from one cylinder to another, first the felt side and then the web side are pressed against the heated surfaces of the cylinders. In addition, hot air may be blown onto the web and between the cylinders to vaporize water from the web. At the end of the dryer section, the completed web typically has a consistency of approximately 1 to 10 percent water and approximately 90 to 99 percent fiber, although other web consistencies are possible at this stage.
The quality of the paper web produced in the papermaking process depends in part on the orientation of the fibers and the consistency of fiber distribution when the embryonic web is formed in the wet-end section of the Fourdrinier machine. The orientation of the fibers within the embryonic web first depends on the distribution of the stock flow to the wire by the headbox. In a pressure forming arrangement of the headbox, the web's boundary layer fibers often become impregnated in the wire. When the web is later transferred from the wire, the boundary layer fibers impregnated in the wire are pulled from the web, leaving small holes in the web. These small holes in the web result in a web that is not as smooth on one side as it is on the other (often called the “phenomena of two-sidedness”). Also, in a pressure forming arrangement, the web's internal layer fibers become forcibly and sporadically misaligned. In a velocity forming arrangement of the headbox, the sheet is formed through a thickening mechanism. This thickening mechanism is due in part to gravitational forces pulling the fibers and the water down through the wire, which causes the bottom-most layers of fibers that settle directly on the wire to be more dense than the upper-most layers of fibers. This high-density layer prevents fibers, fines, and fillers from being pulled through the wire (i.e., higher retention). This high-density layer also prevents water from draining through the wire, resulting in two-sidedness. Both the phenomena of two-sidedness and the disparate orientation of internal layer fibers reduce the quality of the finished paper web.
As water is mechanically squeezed from the paper web in the press section, fines, fillers, and fibers become impregnated in the felt carrying the paper web. The fines, fillers, and fibers plug the felt's water removal channels, resulting in the felt becoming less efficient in removing water from the paper web. As the felt in the press section becomes less efficient in removing water from the web, the dryer section must carry the burden of removing more water from the paper web.
A long-standing problem with papermaking machinery and processes is the large amount of energy required to run the machinery and to produce paper in such processes. A significant portion of this energy is consumed within the dryer section of the papermaking machine. Paper webs having poor fiber formation require significantly more heat to dry than paper webs with good fiber formation and distribution. Therefore, the problems described above regarding fiber misalignment and poor fiber distribution result in paper that requires more energy to dry and that is more costly to produce.
In addition, paper having poor fiber formation is typically lower in machine direction tensile strength when compared with the same grade of paper with a more consistent fiber distribution. This may require expensive chemical additives to increase web strength and can require more sizing, coating, calendaring, and converting operations to produce an acceptable paper product. Improving fiber formation by using more highly refined stock fibers can help to address these issues, but at a significantly increased pulp cost.
In light of the problems and limitations described above, a need exists for a method and apparatus for increasing the quality and manufacturing efficiency of a finished paper web by reducing the phenomena of two-sidedness, improving the distribution of internal layer fibers in the web, lowering the cost of web production through reduced energy requirements, reducing the amount of chemical additives needed for acceptable web strengths, enabling the use of less refined or lower quality stock, improving the retention of fines and fillers within the web, and keeping the forming and press fabrics clean. Each embodiment of the present invention achieves one or more of these results.